The Centurion patent pending C4 rail system is a lightweight, rugged, 2 piece free
float rail that when installed gives the uppers a continuous 12o’clock
rail. The C4 system is the lightest and most rigid rail that clamps to a
standard barrel nut on all AR15, M4, and M16 type weapons systems. This
design allows the installation of the rail on an upper assembly without
removing the front sight post or permanently attached muzzle devices.

I have become very partial to centurion rails. I put this on my other AR and it is phenomenal! Light yet very durable. Clamps tightly onto the barrel nut and runs flush and tight along the upper receiver. Great product and worth every cent. Of course, you get fast shipping along with the product thanks to BCM's commitment to quality customer service.